A sensor typically measures a physical quantity and converts it into a signal that an observer or an instrument can read. For example, a mercury-in-glass thermometer converts a measured temperature into expansion and contraction of a liquid that can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage that a voltmeter can read. For accuracy, sensors are generally calibrated against known standards.
A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1° C., the sensitivity is 1 cm/° C. Sensors that measure very small changes have very high sensitivities. Sensors may also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. The resolution is related to the precision with which the measurement is made.
The output signal of a sensor is typically linearly proportional to the value or simple function (logarithmic) of the measured property. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.